This invention relates to heat-generating bodies which use inductive heating under constant-temperature control.
One known technique for constant-temperature control in inductive heating involves the use of a temperature sensor for detecting the temperature of a heated object, and, based on the detected temperature, adjustment of the electrical power used for inductive heating. In this technique, fitting the sensor may be difficult, and a feedback system for electrical power control may be complex and costly.
Another known technique, illustrated by FIG. 4a, 4b, 7, 11a and 11b, is based on changes in the magnetic permeability characteristics of a magnetic substance at its Curie temperature. In cross section, FIG. 4b shows a copper plate 7 forming an electrically conductive nonmagnetic substrate, and a ferrite laminate 8 of a magnetic material with a specific Curie point. Optionally, a resulting heating element may further include a protective plate 3 laminated to the ferrite. FIG. 4a is a plan view of the heating element.
FIG. 7 is a graph of ferrite magnetic permeability as a function of temperature. As shown in the graph, the ferrite magnetic permeability decreases rapidly at temperatures above the Curie temperature (Tc).
FIGS. 11a and 11b illustrate induced current in an energized state of a heating element placed close to a high-frequency coil 10. As indicated by arrows, currents induced in the copper plate 7 and the ferrite 8 are larger at a lower temperature (FIG. 11a) as compared with a higher temperature (FIG. 11b).
At low temperatures (FIG. 11a), the magnetic resistance in the ferrite 8 is small because its magnetic permeability is large. The alternating magnetic flux is large, and so is the current induced by the magnetic flux in the ferrite. As heat is generated by the induced current, the temperature of the ferrite 8 increases and, due to heat conduction, the temperature of the adjoining copper plate 7 increases also.
With continued heating, the temperature in the ferrite 8 will reach the Curie temperature (Tc), resulting in the high-temperature state shown in FIG. 11b. At this point, the magnetic permeability in the ferrite decreases rapidly, so that the magnetic resistance increases sharply. Because of a sharp drop in the alternating magnetic flux, the induced current decreases rapidly, resulting in a drop in the temperature of the ferrite below the Curie temperature (Tc). By this mechanism, the temperature in the heating element composed of copper plate 7 and ferrite layer 8 oscillates above and below the Curie temperature (Tc), and becomes stabilized at that temperature (Tc), which becomes the average temperature.
In this described technique, use of a ferrite entails high material costs. It is an object of the present invention to provide an efficacious, inexpensive constant-temperature inductive heating element.